Cup anemometer

ABSTRACT

The present invention relates to a cup anemometer having at least two cups ( 1 ) each attached to an arm ( 2 ) of a hub ( 3 ) on a rotary shaft. By cup is meant a generally seen cup-shaped body with a concave inner surface ( 6 ) and a convex outer surface ( 7 ) which meet at the opening of the cup. The opening of the cup is directed essentially along the tangent to the rotational path of the cups. The invention is based on the fact that the cups are truncated at their opening by means of three cuts ( 14, 15, 16 ) which are esentially located in three planes that are parallel with the tangent of the rotational path of the cups at the opening and that, projected on a fourth plane ( 9 ) having the tangent as normal, essentially form a triangle. The cups are attached to their arms ( 2 ) at one of the three corners ( 10 ).

[0001] The present invention relates to a cup anemometer. A cupanemometer is an instrument for measuring the speed of the wind andcomprises a number of, in most cases three or four, cups which areattached to arms of a hub that is allowed to rotate freely about anormally vertical shaft The cups are usually semi-spherical or conical.

[0002] The rotary motion of the cup anemometer is caused by forcesacting on the cups when the current of air blows against the cups. Theforces give rise to moments round the vertical rotary shaft The forcesacting on the cup or cups which essentially has/have their openingtowards the wind are greater than those acting on the cup or Cups whichessentially has/have its/their cupped or pointed side towards the wind.For a certain constant speed of the wind, the number of revolutions willset so as to cause balance between the moments from the driving and thebraking parts. A well designed anemometer rotates at a speed which Isessentially proportional to the speed of the wind.

[0003] The relationship between the speed of the wind and the rotationalfrequency of the anemometer is determined by calibration, in which theanemometer is subjected to known speeds of the wind, in most cases in awind tunnel, over the entire working range. The problem with wind tunnelcalibration is that it cannot be immediately transferred to actualconditions. This is due to the current of air In the tunnel being inmast cases very homogeneously directed and that the current of air haslow turbulence. These conditions are not representative of actual fieldconditions where the wind blows against the anemometer from alldirections at a varying strength (turbulence). The direction of the windvaries not only In the horizontal plane but also In the vertical plane.It has been found that wind speeds measured using anemometers ofdifferent makes under field conditions exhibit great differencesalthough all anemometers were accurately calibrated in a wind tunnel.

[0004] The need for accurate measuring of the speed of the wind isparticularly great in the field of wind energy since the obtainablepower is proportional to the speed of the wind to the third power.

[0005] In the field of wind energy where cup anemometers are used to avery great extent, the speed of the wind has been defined as theresultant of the longitudinal, the transverse and the vertical componentTo be able to measure the wind according to this definition, theanemometer must have a flat angular response. It should also be possiblefor the anemometer to measure the size of the total vector independentlyof at what angle, horizontally as well as vertically, it blows againstthe anemometer.

[0006] All priory anemometers are more or less sensitive to winds havingvertical angles of incidence, i.e. to winds not directed In thehorizontal plane. This causes great problems when measuring the windspeed In the cases where the requirement for measurement accuracy isgreat. No cup anemometer Is currently available that can measure thewind according to the definition as stated.

[0007] If the anemometer does not have a flat angular response, thisalso implies that a possible incorrect mounting of the anemometer, i.e.if it is not mounted fully vertically, results in the measurement of thewind being Incorrect. Moreover the current of air over sloping groundfollows substantially the surface of the ground, and an anemometerhaving a non-flat angular response will under such conditions measure anincorrect speed of the wind.

[0008] The problems described above are solved by the anemometer beinggiven a completely new design, which essentially results in a flatangular response. This is achieved by the cups being given a certaindesign according to that sated in the independent claim. The otherclaims define advantageous embodiments of the invention.

[0009] The Invention will now be described in more detail with referenceto the accompanying drawings, in which

[0010]FIG. 1 shows a prior-art cup anemometer,

[0011]FIG. 2 shows an embodiment of a cup anemometer according to thinvention,

[0012]FIG. 3a shows the geometry of a cup in FIG. 2,

[0013]FIG. 3b is a cross-sectional view of th cup in FIG. 3a,

[0014]FIG. 4a shows an embodiment of a cup with a horizontal sheettherein,

[0015]FIG. 4b Is a cross-sectional view of th cup in FIG. 4a,

[0016]FIG. 5 shows how the circumferential surface of a conical cup,according to a first embodiment of the invention, appears in one plane,

[0017]FIG. 6 shows how the circumferential surface of a conical cup,according to a second embodiment of the Invention, appears in one plane,and

[0018]FIG. 7 shows a diagram of the angular response of a cup anemometeraccording to an embodiment of the invention, compared with the angularresponse of a prior-art cup anemometer.

[0019] The starting point of the present invention is a cup anemometerhaving at least two cups 1 attached to arms 2 which in turn are attachedto a hub 3. The hub is attached to a rotary shaft that Is mounted in aneck 4 of a housing 5.

[0020] By cup Is here meant a generally seen cup-shaped body having aconcave inner surface 6 and a convex outer surface 7. The surfaces meetat the opening of the cup. A special case, which is normally used inanemometers, Involves cups that are rotationally symmetrical about asymmetry axis. When mounted in the anemometer, the symmetry axis of sucha cup is essentially tangent to the rotational path of the cups. It maybe generally said that the opening of the cups is directed essentiallyalong the tangent to the rotational path of the cups. According to theinvention, it is possible to use an arbitrary cup. See the descriptionbelow of a preferred embodiment. An anemometer of this type usually hasthree or four cups. FIG. 1 shows a prior-art such anemometer with threecups 1.

[0021] The invention is based on the fact that the opening of the cup Incombination with its arched part is given a cross-section that varieswith the angle of incidence. When being exposed to the wind, the varyingcross-sections will, like wing profiles of different cross-sections,deflect the current of air in different ways depending on the blowingangle. The deflection varying with the blowing angle causes, accordingto Newton's third law (the law on action and reaction), varying forceson the cups and, thus, varying moments round the rotary shaft. Correctlydesigned, the anemometer can obtain a substantially flat angularresponse.

[0022] A suitable variant to achieve this is to give the cups 1 asubstantially triangular shape of their opening. The triangular shapecan be obtained by the cups being truncated in three cuts that form atriangular pipe.

[0023] If the cup is rotationally symmetrical, as in the special case ofa cone, the truncation causes an arcuate recess in th cup wall, see FIG.3b. The highest point 8 of the arch measured perpendicular from theplane 9 that touches the three corners 10, 11, 12 of the cup should beabout ⅓ of the maximum distance from said plane to the highest point 13of the cup.

[0024] When the truncated cup profile is subjected to a wind blowing ina direction that is normal to one of the cuts, the wind sees an arcuaterecess in the front part and a corner in the rear part. If Instead thecup profile Is subjected to a wind in a direction which is angled atabout ±30° from the normal, and directed in a plane that is generated bythe three corners, the wind sees an arcuate recess in the front part andan arcuate recess in the rear part. These cup profiles that varydepending on the angle of incidence and that are passed by the currentof air cause varying forces and, thus, varying moments round the rotaryshaft.

[0025]FIG. 2 illustrates an embodiment of a cup anemometer according tothe invention with three cups 1. However, it is possible to use adifferent number of cups without deviating from the inventive concept.

[0026] As stated above, the invention is based on the fact that each cupis truncated at its opening by means of three cuts 14, 15, 16 which areessentially located in three planes that are parallel to the tangent ofthe rotational path of the cups at the opening and that, projected on afourth plane 9 that touches the three corners of the cup, essentiallyform a triangle, see FIG. 3a. One of the three corners 10 is attached tothe arm 2 extending from the hub 3.

[0027] Good results have been achieved by means of an embodiment of theinvention where the triangle, seen In said plane, Is essentiallyequilateral. The three corners 10, 11, 12 are conveniently cutoff orrounded. Rounded corners have been found to function very well and areshown in the Figures.

[0028] When manufacturing cups according to the invention, it is easy tomake the desired cuts using a suitable tool, a small circular saw or thelike. Below folly an example describing the manufacture of a templatefor the circumferential surface of a cup that satisfies the requirementsaccording to the invention. With th aid of the template it is easier tomake the cuts In a controlled fashion.

[0029] It has been mentioned above that both semispherical cups andconical cups are known in connection with cup anemometers. The angularresponse for spherical cups tends to be more speed-dependent than forconical cups at the wind speeds concerned. It Is therefore suitable touse conical cups in the invention. An appropriate apex angle to thesymmetry axis of the cone is about 45°.

[0030]FIG. 5 shows, split in a plane, the appearance of a manufacturingtemplate for the circumferential surface of a conical cup which has anapex angle of 45° to the symmetry axis and an equilaterally triangularprojection on a plane having the symmetry axis as normal. The corners amrounded. The edge of the circumferential surface is made up of 7 curvesA-G. The curves are described in local coordinate systems (x,y) with theorigin on the radius R and with the angle φ plotted from the positiveX-axis in a global coordinate system (X, Y). The local coordinatesystems have the y-axis directed radially outwards and the x-axisdirected in the negative φ-direction.

[0031] The curves for the corners have in the local coordinate system(x,y) the formula$\frac{y}{R} = {a*\left( {{\sqrt{1 - {b*\left( \frac{x}{R} \right)}}}^{2} - 1} \right)}$

[0032] and the curves for the sides have in the local coordinate system(x,y) the formula$\frac{y}{R} = {{a*\left( \frac{x}{R} \right)^{1.57}} + {b.}}$

[0033] The constants a and b for each curve are evident from Table 1.TABLE 1 Curve Angle Φ° a b A −37.3 0.2581 19.0 B 5.1 0.3583 −0.3226 C47.6 0.2581 19.0 D 90.0 0.3583 −0.3226 E 132.4 0.2581 19.0 F 174.90.3583 −0.3226 G −142.7 0.2581 19.0

[0034] The location of the curves in the global coordinate system (X,Y)according to FIG. 5 is obtained by the following coordinatetransformations

X=R*cos φ+x*sin φ+y*cos φ

Y=R*cos φ+y*sin φ−x*cos φ

[0035] Successful tests have been carded out with cups having a radius RIn the template which is 46.5 mm.

[0036] The sector S in the Figure is to be excluded when the template isto be rolled up to form a cone. S is calculated as the differencebetween angle φ of curve A and angle φ of curve G. i.e.

S=φ _(A)−φ_(G)=−37.3−(−142.7)=105.4°

[0037] The template gives the limit for the pans that are to be retainedof a pure cone when parts of the edges of the cone at the opening areremoved by machining. The described functions result in a cup that withgood approximation in projection has essentially straight cuts androunded corners.

[0038] A certain degree of further improvement of the flat angularresponse can often be achieved if the outer edge 16 of the cup is notgiven a purely linear projection on a plane having the symmetry axis asnormal but instead somewhat more material of the original complete coneis saved. Correspondingly, a further improvement of the flat angularresponse, above all in the area +10° to +40°, can in many cases beachieved if somewhat more material of the original complete cone issaved at the upper edge 16.

[0039] In such an improved cup, as illustrated in FIG. 6, themanufacturing template of the circumferential surface is formed in thesame way as described above, but having the constants a and b for eachcurve that are evident from Table 2. In FIG. 6, the new curves D′ and F′are indicated by the thick lines while at the same time the previouscurves according to the first variant are indicated by thinner lines forcomparison. TABLE 2 Curve Angle φ a b A −37.3 0.2581 19.0 B 5.1 0.3583−0.3226 C 47.6 0.2581 19.0 D′ 90.0 0.1928 −0.2731 E 132.4 0.2581 19.0 F′174.9 0.1735 −0.2667 G −142.7 0.2581 19.0

[0040] The size of the sector S according to the Figure, which is to beexcluded, is the same as before.

[0041] For the angular response to be as flat as possible, the cups areto be arranged at a suitable distance from the centre of the hub. Forconventional conical cups, the radius, defined as the shortest distancebetween the rotary shaft and the symmetry axis of the cups, should beabout 120% of the diameter of the opening of the cup. In the case ofrotationally symmetrical cups having a triangular opening, the radiusshould be about 95% of the diameter of the circle that encompasses theopening of the cup, i.e. touches the three outermost corners. In cupsaccording to FIG. 5 or FIG. 6, good results have been achieved.

[0042] The rotary shaft of a cup anemometer usually extends in theanemometer upper part, which in most cases is formed as a neck 4. In theneck the rotary shaft is normally mounted, on the one hand in an upperbearing arranged in the upper part of the neck and, on the other hand,in a lower bearing arranged in the lower part of the neck. In the lowerpart, the neck changes n a wider housing 5, which in most casesaccommodates equipment for recording the speed of rotation of the rotaryshaft. Generally seen, the hub 3 and the neck 4 should be long andnarrow and of essentially the same diameter, and the housing 5 should benarrow so as not to unnecessarily interfere with the field of currentround the anemometer. If the upper part of the hub is terminatedabruptly, a lack of equilibrium arises in the field of current and has adetrimental affect on the accuracy. It is therefore convenient to extendthe hub 3 upwards so that essentially current symmetry is obtained inthe area of the hub 3, the arms 2 and the cups 1. The extension 18 canbe farmed as a cylindrical, conical or ellipsoidal extension.

[0043] The effect of the asymmetry in the field of current that remainsin spite of the extension 18 of hub 3 can be minimised by the symmetryaxis of the cups 1 being slightly angled relative to the plane ofrotation of the anemometer so that the openings of the cups pointslightly upwards to the extension of the hub. In the case ofrotationally symmetrical cups, an angle between the symmetry axes of thecups and the plane of rotation of 1-2° Is suitable. For the examples inFIG. 5 and FIG. 8 this provides good results.

[0044] At very large blowing angles to the plane of rotation from aboveand from below, it is difficult to retain a flat angular response withcups as described above. A dear improvement is achieved by placing asheet 17 at the bottom of the cup, said sheet being positionedessentially in the plane of rotation or parallel therewith. The sheetreduces the driving moment for large negative or positive angles ofincidence and thus reduces overspeed of the anemometer for these angles.

[0045] In a preferred embodiment of the invention with rotationallysymmetrical cups, the sheet extends from the point 13 where the symmetryaxis of the cup hits the bottom of the cup, along the symmetry axis toabout 50% of the distance to the plane 9 which has the symmetry axis asnormal and which just touches the corner remotest from the point justmentioned.

[0046] A further advantage of a sheet 17 as described above is that Itserves as a stiffening in the radial direction. As a result, the shapeof the cup, which owing to the centrifugal force tends to be deformed,can be better retained.

[0047] The angular response of a cup anemometer according to FIG. 6 witha sheet according to FIG. 4 is shown in FIG. 7 and is here compared withthe corresponding angular response of a prior art cup anemometer of thetype shown in FIG. 1. The improvement is noticeable.

1. A cup anemometer with at least two cups (1) each attached to an arm(2) of a hub (3) on a rotary shaft, cup relating to a generally seencup-shaped body having a concave inner surface (6) and a convex outersurface (7) which meet at the opening of the cup, the opening of thecups being directed essentially along the tangent to the rotational pathof the cups, characterised in that the cups are truncated at theiropening by means of three cuts (14, 15, 16) which are essentiallylocated in three planes that are parallel with the tangent of therotational path of the cups at the opening and that, projected on afourth plane (9) having the tangent as normal, essentially form atriangle, and that each cup is attached to its arm (2) at one of thethree corners (10).
 2. A cup anemometer as claimed in claim 1,characterised in that the three corners (10, 11, 12) of the cups (1) arecutoff or rounded.
 3. A cup anemometer as claimed in claim 1 or 2,characterised in that the rotary shaft extends in a narrow neck (4)which supports the hub (3) with arms (2) and cups (1), and that the hubhas an extension (18) on its side facing away from the neck, and thatthe hub, as well as the extension in an area adjacent to the hub, has across-section which essentially conforms with the cross-section of theneck in an area adjacent to the hub, so that substantially currentsymmetry is obtained in the area of hub, arms and cups.
 4. A cupanemometer as claimed in any one of claims 1-3, characterised in thatthe cups (1), before being truncated, are rotationally symmetrical.
 5. Acup anemometer as claimed in claim 4, characterised In that the highestpoint (8) of the arcuate recess in the cup wall, measured perpendicularfrom the plane (9) that touches the three corners of the cup, isessentially ⅓ of the maximum distance from said plane to the highestpoint (13) of the cup.
 6. A cup anemometer as claimed in claim 4 or 5,characterised in that the cups (1), before being truncated, are cones.7. A cup anemometer as claimed in claim 6, characterised in that thecones have an apex angle to their symmetry axis that is essentially 45°.8. A cup anemometer as claimed in any one of the preceding claims,characterised in that the triangle seen in said fourth plane (9) isessentially equilateral.
 9. A cup anemometer as claimed in claim 7,characterised in that the circumferential surface of the cones is formedby seven curves A-G being allowed to define a plane surface, of which asector S is excluded and the remaining surface is rolled into a cone,the curves being described in local coordinate systems (x,y) with theorigin on the radius R and the angle φ plotted from the positive X-axisin a global coordinate system (X,Y), and the local coordinate systemshaving the y-axis directed, radially outwards and the x-axis directed inthe negative φ direction and, in the local coordinate system, the curvesfor the corners having the formula$\frac{y}{R} = {a*\left( {{\sqrt{1 - {b*\left( \frac{x}{R} \right)}}}^{2} - 1} \right)}$

and the curves for the sides having the formula$\frac{y}{R} = {{a*\left( \frac{x}{R} \right)^{1.57}} + b}$

and the constants a and b for each curve appearing from the TABLE CurveAngle φ° a b A −37.3 0.2581 19.0 B 5.1 0.3583 −0.3226 C 47.6 0.2581 19.0D 90.0 0.3583 −0.3226 E 132.4 0.2581 19.0 F 174.9 0.3583 −0.3226 G−142.7 0.2581 19.0

and the location of the curves in the global coordinate system (X,Y)being obtained by the coordinate transformation X=R*cos φ+x*sin φ+y* cosφY=R*sin φ+y*sin φ−x*cos φ and the sector S being calculated as thedifference between angle φ of curve A and angle φ of curve G, i.e.S=φ_(A)−φ_(G)=−37.3−(−142.7)=105.4°
 10. A cup anemometer as claimed inclaim 7, characterised in that the circumferential surface of the conesis formed by seven curves A-G being allowed to define a plane surface,of which a sector S Is excluded and the remaining surface is rolled intoa cone, the curves being described in local coordinate systems (x,y)with the origin on the radius R and the angle φ plotted from thepositive X-axis in a global coordinate system (X,Y), and the localcoordinate systems having the yeas directed radially outwards and thex-axis directed in the negative φ direction and, in the local coordinatesystems, the curves for the corners having the formula$\frac{y}{R} = {a*\left( {{\sqrt{1 - {b*\left( \frac{x}{R} \right)}}}^{2} - 1} \right)}$

and the curves for the sides having the formula$\frac{y}{R} = {{a*\left( \frac{x}{R} \right)^{1.57}} + b}$

and the constants a and b for each curve appearing from the TABLE CurveAngle φ a b A −37.3 0.2581 19.0 B 5.1 0.3583 −0.3226 C 47.6 0.2581 19.0D′ 90.0 0.1928 −0.2731 E 132.4 0.2581 19.0 F′ 174.9 0.1735 −0.2667 G−142.7 0.2581 19.0

and the location of the cures in the global coordinate system (X,Y)being obtained by the coordinate transformation X=R*cos φ+x*sin φ+y*cosφY=R*sin φ+y*sin φ−x*cos φ and the sector S being calculated as thedifference between angle φ of curve A and angle φ of curve G. i.e.S=φ_(A)−φ_(G)=37.3−(−142.7)=105.4°
 11. A cup anemometer as claimed inany one of claims 8-10, characterised in that the radius, defined as theshortest distance between the rotary shaft and the symmetry axis of thecups (1), Is about 95% of the diameter of the circle that encompassesthe opening of the cup, i.e. touches the three outermost corners (10,11, 12).
 12. A cup anemometer as claimed in any one of claims 4-11,characterised in that the openings of the cups point slightly upwards tothe extension (18) of the hub, the symmetry axes of the cups forming anangle of 1-2° to the plane of rotation of the anemometer.
 13. A cupanemometer as claimed in any one of the preceding claims, characterisedin that a sheet (17) is placed in the concave interior of each cup andis positioned essentially in the plane of rotation or is paralleltherewith. 14 A cup anemometer as claimed in claim 13 with a cup asclaimed in claim 4, characterised in that the sheet extends from thepoint (13) where the symmetry axis hits the bottom of the cup (1), alongthe symmetry axis to about 50% of the distance to the plane (9) that hasthe symmetry axis as normal and that just touches the corner (10, 11,12) remotest from the point just mentioned.